Ultrasonic transducers are devices that convert energy into sound, typically in the nature of ultrasonic vibrations—sound waves that have a frequency above the normal range of human hearing. One of the most common types of ultrasonic transducers in modern use is the piezoelectric ultrasonic transducer which converts electric signals into mechanical vibrations. Piezoelectric materials are materials, traditionally crystalline structures and ceramics, which produce a voltage in response to the application of a mechanical stress. Since this effect also applies in the reverse, a voltage applied across a sample piezoelectric material will produce a mechanical stress within the sample. Suitably designed structures made from these materials can therefore be made that bend, expand, or contract when a current is applied thereto.
Many ultrasonic transducers are tuned structures that contain piezoelectric (“piezo”) ceramic rings. The piezo ceramic rings are typically made of a material, such as lead zirconium titanate ceramic (more commonly referred to as “PZT”), which have a proportional relationship between their applied voltage and mechanical strain (e.g., thickness) of the rings. The supplied electrical signal is typically provided at a frequency that matches the resonant frequency of the ultrasonic transducer. In reaction to this electrical signal, the piezo ceramic rings expand and contract to produce large-amplitude vibrational motion. For example, a 20 kHz ultrasonic transducer typically produces 20 microns of vibrational peak-to-peak (p-p) amplitude. The electrical signals are often provided as a sine wave by a power supply that regulates the signal so as to produce consistent amplitude mechanical vibrations and protect the mechanical structure against excessive strain or abrupt changes in amplitude or frequency.
Typically, the ultrasonic transducer is connected to an ultrasonic booster and a sonotrode (also commonly called a “horn” in the ultrasonic welding industry), both of which are normally tuned to have a resonant frequency that matches that of the ultrasonic transducer. The ultrasonic booster, which is structured to permit mounting of the ultrasonic transducer assembly (or “stack” as it is commonly called), is typically a tuned half-wave component that is configured to increase or decrease the vibrational amplitude passed between the converter (transducer) and sonotrode (horn). The amount of increase or decrease in amplitude is referred to as “gain.” The horn, which is oftentimes a tapering metal bar, is structured to augment the oscillation displacement amplitude provided by the ultrasonic transducer and thereby increase or decrease the ultrasonic vibration and distribute it across a desired work area.
Typically, all of the mechanical components used in an ultrasonic transducer assembly must be structured so that they operate at a single resonant frequency that is near or at a desired operating frequency. In addition, the ultrasonic transducer assembly must often operate with a vibrational motion that is parallel to the primary axis (i.e., the central longitudinal axis) of the assembly. The power supply for the stack generally operates as part of a closed-loop feedback system that monitors and regulates the applied voltage and frequency.
For certain applications, particularly those involving welding of thermoplastic parts together, ultrasonic welding technology is highly desirable due to its consistency (particularly when the stack's movement is controlled by a servo-driven motor), speed, weld quality, and other advantages. However, a phenomenon known as “read-through” can occur particularly when one of the parts is thin. Read-through in this context is a visual artifact that can be produced during the welding of one side of a thin part, which is visible by the naked eye from the opposite side of the thin part. For applications where the thin part is a highly visible feature of a manufactured system, read-through is highly undesirable. This is especially the case when the thin part is painted or has a high-gloss finish, when visible artifacts in the form of read-through become especially pronounced. The welding process disturbs the original unblemished state of the thin part in the form of bumps, wavy artifacts, and other perturbations in the exposed surface of the thin part, which are even more pronounced when the thin part has a large, smooth surface.
It has been observed that read-through occurs when the thickness of the area to be welded to another part is 2.8 mm or less and the frequency of the ultrasonic energy applied to the part is 40 kHz or less. Welds produced by these methods produce undesirable read-through on the finished surface of the thin part to which the part is welded. As the thickness of the thin part decreases, the read-through effect becomes increasingly pronounced and unacceptable.
A need exists, therefore, for a solution to this read-through problem. Aspects of the present disclosure are directed to fulfilling this need by eliminating read-through, among other objectives.